MXR Micro Amp Pre Dunlop vs. New

[schenkadere]

I have a 70's pre Dunlop, pre LED Micro Amp (block logo). I was wondering if anyone has experience with the newer version and how they compare. Mine is very hissy...are the newer ones as well?

[Mark Hammer]

The original used a TL06x op-amp. The 06x series (061, 062, 064) have lower current consumption than the 07x series (o71, 072, 074). Unfortunately, they also have higher noise as a tradeoff. That's one of the reasons why some of the Behringer clones of Boss and other pedals have garnered some comments about noise; they chose to use a lower-current, rather than lower-noise, chip.

If you swap out the chip in there for the lower-noise version, you'll be pleased with the performance. Besides, the original did not assume a pedal-board and external supply, so they leaned in favour of increasing battery life. If you have a pedal-board and wallwart supply, you don't have to pursue that same bias.

[schenkadere]

Wow...very informative...thank you! Any comments on tone qualities?

[Mark Hammer]

There should be no difference in tone quality. If, for some reason there is, I wouldn't expect it to result from chip type differences any moreso than chip to chip differences within type (i.e., this TL061 stands as much of a chance of being different sounding than that TL061 as it does being different than that TL071).

[Elvisofnazareth]

I had a Block Logo with no AC jack from 1980 (?)

The LED responded with your picking, it was one of my all time favorite pedals.

[Mark Hammer]

I had a Block Logo with no AC jack from 1980 (?)

The LED responded with your picking, it was one of my all time favorite pedals.

That is weird. I can recall of no issue of an MXR floor pedal that provided any sort of LED monitoring of level. You sure it wasn't modded?

[Elvisofnazareth]

"That is weird. I can recall of no issue of an MXR floor pedal that provided any sort of LED monitoring of level. You sure it wasn't modded?"





Oh yeah?
That's pretty likely. I bought it off a guy named Roy Page down in Vegas who I sold some plexi transformers to. He had some great pedals including an old tan Ross that he modded, It sounded amazing.

I just located and shot him an email to find out if it was a mod..

It was very cool, It seemed to only flicker when it was adding a little crunch.
Very smooth, not fizzy at all.

[schenkadere]

My micro Amp didn't have the LED monitoring...had to be a mod.

I sold mine and got a TC Electronic Vintage Tube Primer(same a T Rex Moller OD). Much better and quieter clean boost plus a great TS808 clone in one pedal...125 bucks brand new on eBay! Amazing pedal.

[Elvisofnazareth]

I'm waiting to hear back from Roy still, maybe I can get the info on his mod and the O.K. to post it if anybody is interested.

I can't believe vintage Micro Amps can still be found so cheap. It's a sleeper.

[echodeluxe]

My micro Amp didn't have the LED monitoring...had to be a mod.

you are correct. those old micro amps didnt have leds. however, the pedal wasnt modded for "input monitoring", someone just wanted to add an led and wired it in wrong.

what im assuming happened was they wired the led across the input/output, not realizing they had to wire it to 9v with a resistor for current limiting. so what happens is whenever you feed a hot signal to the pedal, the led will flash, depending on the voltage going in.

why do i think this is what happened? because i own a 1980 micro amp were someone did just that

[Sweetfinger]

Wow...very informative...thank you! Any comments on tone qualities?

Before Dunlop reissued them, I built a couple for my own use. All, including the original that I cloned, got weird in uppermost gain settings. Shrill is too strong a word, but is the only adjective I can think of. Lower to mid-gain settings sounded fine, and I never really found myself using them in the upper range anyway. One pedal was a bit warmer, but probably due to the fact that I used junk-box caps and resistors. I had to parallel some components and use others that "worked" but weren't quite the same values. FWIW I used TL071's- Batteries seem to last forever in mine so I don't think current drain is an issue.

[Mark Hammer]

My gut sense is that any "shrillness" is not a result of the Micro Amp directly. It does trim back a bit of the bass as gain is increased, but not very much at all, and certainly not enough to produce the sort of stridency one hears in the Distortion+ (which also trims back bass as gain goes up, though much more).

More likely, the shrillness is really a product of the failure of the unit to provide any means to take treble away form the original signal. Keep in mind that as you push the front end of the amp harder, you generate additional harmonic content. If part of what you push the amp with is itself harmonic content, then you generate harmonics of harmonics, resulting in a shrieky and unpleasant tone. "Good" overdrive requires that the signal be preconditioned to have its edges rounded off before smacking the amp upside the head. The Micro-Amp just makes the signal louder, without any systematic alteration or compensation of the signal. Consequently, while it doesnot make your amp misbehave with modest gain settings, when the u-Amp is cranked, it extracts the worst from your amp.

[Sweetfinger]

My gut sense is that any "shrillness" is not a result of the Micro Amp directly. It does trim back a bit of the bass as gain is increased, but not very much at all, ....More likely, the shrillness is really a product of the failure of the unit to provide any means to take treble away form the original signal.

As I said, "shrill" was too strong a word, but what you're describing above is exactly what occurs, it trims back the bass slightly as gain is increased.

[Mark Hammer]

Any detectable loss of bass is a by-product of the way that gain is adjusted.

In a non-inverting op-amp configuration (which is what the Micro-amp uses), there will be a resistance and cap in series, and going to ground, from the inverting ("-") pin. Making the resistance smaller increases gain, by dumping feedback signal to ground. But as you decrease the resistance, the specific content dumped to ground changes.

Using the formula F = 1/(2*pi*R*C), a 4.7uf cap, 2k7 fixed resistor, and 500k pot set to max resistance, produces a low-frequency rolloff at just under 1hz. In contrast, when the pot is set to min resistance (zero ohms), the recalculated rolloff is still pretty low, 12.5hz. So, it would seem the Micro-Amp, in theory at least, does not lose bass as much as as produce too much treble.

The upper-end rolloff is a function of the feedback resistor and cap; in this case 47pf and 56k, which yields a top-end rolloff at just over 60khz. Putting both rolloffs together, we see that the Micro-Amp does nothing to deliberately constrain either top or low end; an essentially "flat" booster.

A couple of years ago, I posted a design for a booster called The Crank (http://hammer.ampage.org/files/The_Crank.gif) which attempted to tailor the frequency response at higher gains so that there was not as much low bass and not as much treble pushing the amp; especially top end. I accomplished this by adjusting the gain using both the "ground leg" (as the Micro-amp does) AND the feedback path (as the Tube Screamer does) at the same time, using a dual-ganged pot. Works great.